How to Find the Defective Battery in Your Battery Bank

It only takes one bad battery to bring your off grid power system to a complete halt. Even in systems that have 8, 16, 24 or even more batteries, one defective battery can drain every good battery and leave you powerless. Unfortunately it is not always easy to find the battery causing all the trouble. However, if you follow the steps in this article, you will be able to pick out the bad apple within an hour or so.

The story is usually the same. You have been living off the grid for awhile and although you don’t have unlimited power, you are learning to live with what you got. But lately things aren’t working as well. You don’t think you are using more power than usual and you think your solar modules, wind turbine, water turbine or generator are working normally. But it seems like the batteries are always low, or the voltage drops off very fast even when the batteries should be full. Time to check your battery bank.

Table of Contents

Some of the signs you might have a defective battery.

Poor Performance – The most common symptom of a bad battery is that your power system is not working as well as it used to. After months of operating your power system you will get good at predicting battery voltage and how much capacity you have. When a battery starts to cause problems, the voltage will be lower than usual or your bank will have less capacity than it should. Sometimes this decline in performance is very gradual (more difficult to diagnose) and sometimes it is immediate.

Excess Heat – When opening your battery box it is common for a buildup of heat when a battery is failing. The temperature inside your box is another factor you will get good at estimating. Most of the time your batteries will remain cool but in times of heavy charging or heavy discharging they will make more heat. A defective battery will sometimes make heat all the time. If a cell is shorted inside the battery it will make heat 24 hours a day and discharge your other batteries producing even more heat.

Requires More Water – Another common sign of a bad battery is the requirement to add more distilled water than usual. Sometimes only the defective battery requires more water and sometimes the whole bank will require more water than usual.

How to diagnose the defective battery fast.


Before disconnecting anything, take a look inside your battery bank.

Do you see:

  • any leaking batteries? Any electrolyte in the bottom of your battery box?
  • excess moisture on top of any battery?
  • excess dirt on top of any battery?
  • excess corrosion on any battery?
  • any signs of heat on any battery?
  • are any of the batteries swelled up or expanded?

Defective batteries usually gas more than good batteries forming extra dirt, electrolyte or corrosion on top of the battery. If you see one battery with any of the above signs, you have probably found the bad battery already. If all of the batteries seem the same, proceed to the next step.


Next feel each battery. Are any of them hotter than the others? The hot one will likely be the damaged battery. When a battery is shorted inside it might make heat all the time. It is also possible for a shorted battery to only make heat intermittently. If one of your batteries is much warmer than the others, it is a definite sign something is wrong.


Remove the caps. Are any of the battery’s electrolyte lower than the others? Sometimes there will only be one faulty cell. You must look inside all the cells of every battery. Any battery that is low enough that the plates are exposed is a problem. If one battery is only down a little and the plates are all covered, it is possible the battery is beginning to fail. If all the batteries (or the majority) are down, it is simply time to add distilled water. Learn about adding water and good battery maintenance tips here.

If you find a battery that has one or more low cells (compared to all the others), you have likely found the bad battery. If a battery is low, take a little extra time to look for a crack in the housing (container) of the low battery. Sometimes there is a tiny crack right at the level of the electrolyte. It might not be leaking anymore as the electrolyte is now below the crack.


If all the batteries have plenty of electrolyte, get a good flashlight and look down into the cell of each battery.

  • Do the plates looks clean?
  • Are any of the plates swelled up or damaged? (As in photo below)
  • Do any of the plates have more sulfation (white coating) than the others?
  • Is it black inside (more than the others), or does it look burnt more than the others?
  • Is the electrolyte clear or really dirty?

It is important to note that batteries won’t always look like new inside. There will be dirt and the electrolyte will look burnt over time. What you are really looking for is inconsistency between batteries or cells. If all cells of all batteries are dirty, then all is well. But if only one or two batteries are dirty inside, you have likely found the problem(s).


Check the specific gravity (S.G.) of the electrolyte of each cell of each battery. Get a good quality, temperature compensated battery hydrometer for this test. Like the previous tests you are looking for inconsistency. The S.G. will vary constantly depending on the depth of discharge of your batteries, the amount of electrolyte in each cell, how long it has been since your last equalization charge and how long it has been since you added distilled water.

If you have a problem battery, the specific gravity will be much different than the others. Usually the defective battery will have an unusually low S.G. but an unusually high reading may indicate a problem as well. Even one cell with an unusually low S.G. is enough evidence to indicate a problem battery.


With all the batteries still connected by the cables, check the voltage of each battery while they are at rest. Any significant differences?

While applying a charge to the battery bank, check the voltage of each individual battery. A faulty battery will usually show a higher voltage than the others while being charged. The higher voltage is a result of the battery not having as much capacity as the others.

While discharging the battery bank, check the individual voltage of each battery. A troubled battery will show a lower voltage than the other batteries. This lower voltage is also due to the reduced capacity.

If you get a strange voltage reading during charging or discharging, it is very important the you take a good look at the surrounding battery cables and lugs.

  • Are they tight?
  • Do they feel hot or look like they have been hot or burnt?
  • Are the cable lugs simply crimped or soldered as well?

It is VERY COMMON to have a defective battery cable/cable lug that will act as if the battery itself is the problem. If the lug was only crimped, many years of being heated and cooled can cause the lug to become loose. A loose cable/lug will decrease current flow to the surrounding batteries and make the battery appear to be defective. A loose cable lug can also make a lot of heat, so much heat it can melt the cable or the terminal on top of the battery. A loose connection can act like an electric “stick welder” and either fuse itself together or melt everything.

In the above photo, the crimped on lug on the interconnect cable was loose. The loose lug got hotter and hotter until it was hot enough to melt the lead battery post. Strangely enough you will see a nice, brand new cable attached to the melted stud/post. This was an attempt to fool the battery supplier into warrantying the golf cart battery for a defective post. Any experienced battery installer/supplier would not be tricked by this. You can see the nice heat shrink (which would have melted off), a clean looking lug (which would normally be discolored from the heat) and the melted indent on the top of the battery from the faulty cable.


If you have not found the problem battery yet (which would be very unusual), it is now time to remove all the battery cables and interconnect cables.

Before removing the cables, do your best to charge the battery bank as much as possible. If the battery bank has been sitting around for awhile without being charged it is possible the bad battery will lower the voltages and specific gravities of the good batteries.


With all cables removed, check the voltage of each battery. A battery with a shorted cell will have its voltage drop very fast when disconnected from the rest of the bank. When connected to the rest of the batteries, they will support the bad battery and hold the voltage as long as possible. Once the battery is isolated it cannot hold its voltage for more than a few minutes.

If all the voltages are stable, then it is unlikely you have a shorted cell. It doesn’t mean your batteries are all good. It just means your battery is not short circuited inside.


Check the specific gravities of all of the cells of all of the batteries as in STEP 5. Any wild variation will tell you there is a faulty battery/cell. The faulty battery will likely have a very low S.G. when removed from the rest of the pack.

STEP 10:

Now you will need to remove your batteries from the box so you can have a good look at each of them. Does the case (plastic container) look like it is swelled or expanded? It will look like someone hooked it up to an air compressor and over filled it. Any battery that is swollen has little, if any, life left.

Repeat STEP 1 and STEP 3 now that the batteries are out of the box and much easier to look at.

If all of this work has left you frustrated by the fact you haven’t found any problems yet, be patient, you have not tried everything yet.

STEP 11:

The last step is to load test each individual battery until you find the defective unit. You can obviously take your batteries to a professional battery installer/supplier and have them perform the tests but that is a lot of heavy work.

Instead you could do some crude load testing yourself.


If your batteries are 12 volt units it is pretty easy to find 12 volt loads such as light bulbs or heaters. RV and automotive stores have lots of 12 volt light bulbs.

You should be able to find 25, 50, 100 and 200 watt 12 volt bulbs for your tests.

Each 100 watts of 12 volt light bulbs should remove about 8 amps from your 12 volt battery.

Using the manufacturer’s literature find the C5 rating for your battery and divide by 5. This is the amperage of our load testing load.

For example:

Your battery is C5 rated 100 amp hours at 12 volts.


100 AH / 5 = 20 AMPS (at 12 volts) = 240 WATTS

For this test we would need two 100 watt, 12 volt light bulbs and one 50 watt, 12 volt light bulb.

Now test each battery by applying the load (light bulbs) and monitoring the battery voltage. If you have unlimited time you could keep the load connected until the battery reaches 10.5 volts just like the manufacturers. However you could also keep the load connected for 30, 60 or 90 minutes, record the battery voltage, and compare to the other batteries in your bank. The faulty battery will see a much more drastic voltage drop then the others.


Load testing 6 volt batteries is a little more complicated as 6 volt loads aren’t that readily available.

You have two options:

  1. Test two batteries at a time in series and test just like a 12 volt battery
  2. Make 6 volt loads from 12 volt loads

Using two batteries in series is doable but it is a hassle. First you would test all pairs of batteries as above. One or two pairs will likely be underperforming. If you have a pair that underperforms you will  now have to test each of those batteries. The problem is they will probably be dead and have to be charged, as well as, two more batteries to pair with each of the potentially defective batteries. It can be frustrating. We prefer making 6 volt loads from 12 volt loads.

As stated above, 6 volt loads are not easy to find. However you can use 12 volt loads and de-rate them to 6 volts.

Any load designed for 12 volts will use 1/4 the rated power at 6 volts.

Why 1/4 and not 1/2 the power? Read this article about using 120 or 240 volt heaters as dumps loads as it explains how a resistor/heater consumes power at different voltages.

This time we will use a Trojan T105 as our battery of choice. The C5 AH rating for a T105 is 185 amp hours at 6 volts.


185 AH / 5 = 37 AMPS = 222 WATTS

As we are using 12 volt bulbs we need to multiply 222 watts by 4 to make up for the de-rating of the 12 volt bulbs at 6 volts.

Bulbs required  = 888 watts at 12 volts

For this test we would need 8 100 watt,  1 50 watt, and 1 25 watt 12 volt light bulbs. If you don’t want to mess around with light bulbs, buy a resistor, 12 volt hair dryer or 12 volt car heater.

Connect your new load to each battery for a set amount of time. It could be 30, 60, 90 minutes or even a few hours.

Monitor the voltage of each battery and discard the battery or batteries that do not perform.